1. Field of the Invention
The present invention relates to an epitaxial growth process for compound semiconductor crystals and more particularly to a process wherein a crystal, such as Ga.sub.1.sub.-x Al.sub.x As (wherein 0&lt;x.ltoreq.1), on which an inactive layer is easily formed by chemical reaction with a small amount of impurities contained in surrounding gas due to its chemical activity, or a crystal, such as GaP and GaAs, on which a degenerated layer is formed due to high vapor pressure of its constituent elements, is used as a substrate crystal.
2. Description of the Prior Art
A crystal of GaAs or GaP is widely utilized as a substrate in epitaxial growth processes for III-V compound semiconductor crystals in liquid phase, such as GaP, GaAs, Ga.sub.1.sub.-x Al.sub.x As (0&lt;x.ltoreq.1), and Ga.sub.1.sub.-x Al.sub.x P (0&lt;x.ltoreq.1). In these processes, it is necessary to heat a crystal growth system consisting of a substrate crystal and a solution for crystal growth to a predetermined temperature and to keep it at that temperature during a period of time before the beginning of crystal growth so that the solution for crystal growth is well equilibrated and homogenized. During this period of time, the substrate crystal is exposed to the surrounding gas at a high temperature. In this case, when the substrate is a Ga.sub.1.sub.-x Al.sub.x As monocrystal, its surface is covered by an inactive layer, which seems to be an oxide, due to its chemical activity, and therefore it is not possible to obtain an epitaxial layer of high quality. In case the substrate is a monocrystal of GaAs, GaP, Zn Te, or Zn Se, since the dissociation pressure of one component is high at a high temperature, a degenerated layer is formed on the surface of the monocrystal by dissociation of As, P, Zn, Te, or Se at temperatures around the temperature for crystal growth (600.degree..about.1,000.degree.C.). When the interface between the substrate and the epitaxial layer is used as an active region, for example a p-n junction, the degenerated layer lowers the stability of the active region. Moreover, the degenerated layer has bad influences on the crystallographical properties of the layer grown on it.
In order to overcome this difficulty, several methods have been already proposed. According to one of these methods, after having brought a substrate crystal in contact with a solution for crystal growth, the temperature of the system for crystal growth is raised until the solution becomes subsaturated. The degenerated layer formed at the surface portion of the substrate crystal is dissolved by the subsaturated solution. After that, the temperature is again lowered so that a crystal begins to grow on a clean surface of the substrate. This method has a drawback that, since impurities attached on or included by the substrate crystal are dissolved by the solution together with the constituent of the crystal and mixed with the solution for crystal growth, they are in turn distributed in the grown layer and influence disadvantageously the characteristics of the active region formed in the grown layer or between the substrate crystal and the grown layer.
According to another method, apart from a solution for crystal growth, an etching solution is prepared. After having brought a substrate crystal in contact with the etching solution, a system for crystal growth consisting of the substrate crystal and the two solutions is heated to a temperature for crystal growth. The etching solution becomes subsaturated just as mentioned previously and begins to dissolve a degenerated layer covering the surface of the substrate crystal. After that, the substrate crystal is brought in contact with the solution for crystal growtn so that a crystal begins to grow on a clean surface of the substrate crystal (Japanese patent application No. 46-101899). This method removes the drawback of the first method mentioned above. However, since the substrate crystal is kept in contact with the etching solution during the heating process of about one hour for dissolving the degenerated layer on the surface of the substrate crystal, this method has another drawback that reaction-limited dissolution takes place, by which more easily dissolvable parts, such as parts containing a number of crystallographical defects, of the surface of the substrate crystal are selectively dissolved so that one cannot obtain a flat surface of the substrate crystal. Another disadvantage, which is common to the two methods described above, i.e. to the methods according to which one heats a substrate crystal and an etching solution kept in contact with each other, is that it is difficult to find an optimum condition for obtaining a flat surface, because the dissolution of the surface of the substrate crystal depends considerably upon variations of the speed of the temperature rise, differences of the temperature distribution, etc.
According to still another method, a layer, which is not used as active region, is grown on a substrate crystal and another layer, which is used as an active region, is grown thereon by a multilayer successive growth method. This method has a disadvantage that a device for crystal growth is complicated, because impurities added to a solution for crystal growth, S, Se, Te, Zn and the like for III-V compound semiconductors and Zn, S, Te and the like for II-VI compound semiconductors, for which Bi, In, or Sn is used normally as solvent, are easily vaporized due to their high vapor pressure at high temperature and mixed with other solutions so that the solutions influence each other.